'Grimm': Kate del Castillo is on the Case of the Weeping Woman (Exclusive Video)

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The "Weeds" alum guest stars in Friday's Halloween episode as a mysterious detective.

NBC's Grimm is ready to add some creepiness to Halloween.

The network's Friday night sci-fi procedural will tackle the case of the Weeping Woman with Weeds alum Kate del Castillo for the episode titled "La Llorona," which is loosely based on the famed legend of a woman who drowned her children.

In a clip from the episode exclusive to The Hollywood Reporter, del Castillo plays Valentina Marquez, a mysterious detective from Albuquerque who comes to Portland to help Nick (David Giuntoli) and Hank (Russell Hornsby) capture a serial child abductor. Valentina has spent the past five years tracking the perp and identifying patterns in related abductions -- which include the drowning deaths of two boys and a girl between the ages of 7 and 10, with little to no evidence left behind.

According to the La Llorona (which translates to weeping woman) legend, a woman named Maria kills her children by drowning them in order to be with the man she loves. Upon his rejection, she commits suicide but is not permitted to enter the afterlife until she finds her missing children, forcing her to wander the Earth for all eternity, crying, as she searches for her drowned offspring.